Make sure that the units are properly grounded (electrically). You mentioned about reversal, but try setting the signaling to just loop start, not line reversal.
The ring frequency has nothing to do with the PSTN side. It is used for the pbx side. And the pstn ring sensitivity is used for calls that come from PSTN to tenor.
Make sure that you are at the latest software version on the unit (latest P4-2-20 should be fine).
Other than that, I know that early on there was a lot of problems like this with gsm devices and there was never a full solution. As of late, I really have not seen this problem come up.